Smoke detectors detect the presence of smoke particles as an early indication of fire. Smoke detectors are used in closed structures such as houses, factories, offices, shops, ships, aircraft, and the like. Smoke detectors may include a chamber that admits a test atmosphere while blocking ambient light. A light receiver within the chamber receives a level of light from an emitter within the chamber, which light level is indicative of the amount of smoke contained in the test atmosphere.
Several types of fires need to be detected. A first type is a slow, smoldering fire that produces a "gray" smoke containing generally large particles, which may be in the range of 0.5 to 1.2 microns. A second type is a rapid fire that produces "black" smoke generally having smaller particles, which may be in the range of 0.05 to 0.5 microns. Fires may start as one type and convert to another type depending on factors including fuel, air, confinement, and the like.
Two detector configurations have been developed for detecting smoke particles. Direct, or obscuration, detectors align the emitter and receiver such that light generated by the emitter shines directly into the receiver. Smoke particles in the test atmosphere interrupt a portion of the beam thereby decreasing the amount of light received by the emitter. Obscuration detectors typically work well for black smoke but are less sensitive to gray smoke. Additionally, obscuration detectors typically are not within a chamber, as they have an emitter and a receiver spaced at a substantial distance, such as one meter or across a room, whereas smoke detector chambers are preferably located within a compact housing. Indirect or reflected detectors, commonly called scatter detectors, have an emitter and receiver positioned on non-colinear axes such that light from the emitter does not shine directly onto the receiver. Smoke particles in the test atmosphere reflect or scatter light from the emitter into the receiver. Reflected detectors generally work well for gray smoke but have a decreased sensitivity to black smoke.
Smoke detectors typically use solid-state optical receivers such as photodiodes due to their low cost, small size, low power requirements, and ruggedness. One difficulty with solid-state receivers is their sensitivity to temperature. Additional circuitry that increases photo emitter current with increasing temperature partially compensates for temperature effects. Typical detectors also require complicated control electronics to detect the light level including analog amplifiers, filters, comparators, and the like. These components may be expensive if precision is required, may require adjustment when the smoke detector is manufactured, and may exhibit parameter value drift over time.
What is needed is a smoke detector with good sensitivity to both gray smoke and black smoke. The smoke detector should use a minimum of analog components to reduce cost and the possibility for component value drift over time. The smoke detector should also compensate for the effects of ambient temperature.